Total knee arthroplasty (TKA), commonly known as total knee replacement, is one of the most frequently performed orthopedic procedures worldwide, offering relief from debilitating pain caused by conditions like arthritis. While the surgery is highly successful, a rare but serious complication is injury to the nerves surrounding the knee joint. The overall risk of a major nerve injury is very low, typically cited in the range of 0.12% to 0.4% in primary TKA procedures, though certain risk factors can increase this possibility. Understanding the specific symptoms of nerve damage is important for patients and healthcare providers, as timely recognition is crucial for the best possible recovery.
Identifying the Specific Symptoms
Nerve damage symptoms after knee replacement are categorized into sensory and motor deficits, depending on the specific nerve affected. The most commonly injured nerve is the common peroneal nerve, which wraps around the outside of the knee near the fibula head. Injury to this nerve often results in a combination of sensory changes and muscle weakness.
Sensory symptoms manifest as an altered feeling in the lower leg and foot. Patients may experience numbness, painful hypersensitivity, tingling, burning, or the “pins-and-needles” feeling known as paresthesia in the skin of the lateral calf and the top of the foot. These sensory changes can range from mild annoyance to severe, chronic pain.
The most noticeable motor symptom associated with common peroneal nerve injury is “foot drop,” the inability to actively lift the front part of the foot (dorsiflexion). This weakness causes the foot to drag on the ground while walking, often leading to tripping and a characteristic high-stepping gait. Less frequently, the tibial nerve may be affected, resulting in weakness in the calf muscles responsible for pushing the foot downward (plantarflexion), or numbness on the sole of the foot.
Sensory nerves near the incision, such as the infrapatellar branch of the saphenous nerve, are almost always cut during the standard surgical approach. This expected injury causes a patch of numbness on the front and inside of the knee, which generally resolves over time. However, the severed nerve ending can sometimes form a painful scar or neuroma, leading to persistent, localized pain.
Mechanisms of Nerve Injury
Nerve damage during knee replacement results from mechanical or ischemic factors during the surgical process. One primary mechanism is direct trauma, where the nerve is stretched, compressed, or cut by surgical instruments or retractors. This risk is particularly high when a severe pre-existing knee deformity, such as a significant valgus (knock-kneed) angle, requires substantial correction to realign the limb.
The rapid correction of a severe deformity places excessive tension on the common peroneal nerve, which is already under stretch. This intraoperative traction damages the nerve fibers, a process often exacerbated by the nerve’s fixed position as it wraps around the fibular head.
Additionally, the use of a tourniquet to control blood flow during the procedure can lead to nerve injury. This damage is caused by direct mechanical compression or a lack of blood supply (ischemia), especially if the tourniquet time is prolonged beyond two hours. Post-operatively, nerves can also be compressed by internal factors, such as significant swelling, a large hematoma (blood clot), or constrictive dressings. These secondary causes of compression can cut off the nerve’s blood supply, leading to delayed symptoms that appear days after the surgery.
Confirming the Diagnosis
When a patient exhibits symptoms suggesting nerve involvement, a healthcare provider begins with a detailed physical examination to pinpoint the location and extent of the deficit. This includes testing specific reflexes, assessing motor function by asking the patient to move their ankle and toes against resistance, and mapping areas of sensory loss to determine which nerve pathway is affected. The timing of symptom onset is also considered: immediate symptoms are more likely related to mechanical trauma, while delayed symptoms may suggest compression from swelling.
To confirm the diagnosis and assess the severity of the injury, specialized electrodiagnostic tests are typically ordered. The two main tests are Nerve Conduction Velocity (NCV) studies and Electromyography (EMG). NCV studies involve placing electrodes on the skin to deliver small electrical impulses and measure how quickly and strongly the nerve transmits the signal. A slow or absent signal indicates damage to the nerve’s insulating myelin sheath or the fibers themselves.
EMG involves inserting a fine needle electrode into the affected muscles to record their electrical activity. This test helps determine if the nerve is successfully sending signals to the muscle and estimates the degree of nerve damage. While these tests are not performed immediately, they become valuable several weeks after surgery to track nerve function and recovery, especially if symptoms persist beyond three months.
Treatment Pathways and Recovery
The initial management for suspected nerve injury involves non-surgical, conservative measures, as many mild nerve palsies resolve spontaneously. The surgeon may remove any constricting dressings and ensure the knee is positioned in slight flexion, which reduces tension on the common peroneal nerve. Physical therapy is immediately implemented to maintain joint range of motion and prevent muscle contractures while waiting for nerve function to return.
For patients experiencing foot drop, an ankle-foot orthosis (AFO) brace is commonly prescribed to stabilize the foot and ankle, allowing for safer walking. Medications are often used to manage neuropathic pain, characterized by burning or shooting sensations. These include drugs like gabapentin or pregabalin, which calm the overactive pain signals transmitted by the damaged nerve.
If motor deficits show no sign of recovery after three months, or if electrodiagnostic studies indicate a severe, non-recovering injury, surgical intervention may be considered. This can involve nerve decompression, where the surgeon releases the nerve from scar tissue or external compression, or, in rare cases of severe injury, a nerve repair. Motor recovery, if it occurs, typically begins within a few months, and complete or near-complete recovery is often achieved within one year following the injury.